Method of producing refractory metal articles



United States Patent Office 3,446,655 Patented May 27, 1969 US. Cl. 117-129 9 Claims ABSTRACT OF THE DISCLOSURE A refractory article formed of a metallic material composed of at least 50% by weight of niobium or tantalum has its surface protected by a mixed metallic oxide glaze composed of 30% to 50% silicon, vanadium and/or manganese to 25 and at least three of the following: tantalum, niobium, tungsten, molybdenum, chromium, titanium, zirconium, aluminum, and boron.

The invention relates to refractory articles composed principally of niobium or tantalum, or of an alloy comprising at least fifty percent by weight of one of these metals, and having a surface modified to reduce interstitial penetration at high temperatures by gases such as oxygen, nitrogen and hydrogen.

According to the invention the surface is protected by an outer surface layer which is a mixed metallic oxide glaze, the metal constituents of which are the following metals within the ranges specified (reckoned by weight on the total metal content of a sample of the glaze):

(a) Silicon, 30 to 50%;

(b) Vanadium or manganese 5 to 25 or both vanadium and manganese totalling 10 to 25% (c) At least three of the following:

Percent Tantalum 5 to 50 Niobium 5 to 50 Tungsten 3 to 25 Molybdenum 3 to 25 Chromium 3 to 25 Titanium 3 to 25 Zirconium 3 to 25 Aluminium 3 to 25 Boron 3 to subject to the limitation that the total of tantalum and niobium does not exceed 50%.

(The term metal is used in this specification to include silicon and boron.)

By providing the outer surface layer in the form of a glaze, better resistance to interstitial penetration by gases such as oxygen, nitrogen and hydrogen is obtained, than with an unglazed outer surface layer, but it is necessary for the glaze to be resistant to de-vitrification. In general, the more complex the constitution of the glaze, the more resistant it is to de-vitrification, which is important for glazes exposed to attack by de-vitrification at low temperatures, say 1100 C. or less. A reasonable degree of resistance is obtained by including at least five elements. The presence of silicon and either vanadium or manganese (or both) is particularly advantageous when, in accordance with a preferred method of making the article, a sintered layer of the elements in metallic form is first formed on the article and is then oxidised and glazed, since the silicon, vanadium and manganese form a liquid phase of relatively low melting point which aids in sintering the other constituents to form a strongly adherent coating. The subsequent presence of vanadium and/or manganese oxides in the surface layer has also been found to assist the formation of a homogeneous glaze.

In one preferred method of making articles according to the invention, an adherent coating of the constituent metals of the glaze, or at least of those not already present in the body of the article, is applied to the surface of the article by dipping in a slurry or by a cold or hot spraying process, and the article is then heated at least once under non-oxidizing conditions to fuse the coating onto the article and then under oxidizing conditions to oxidise and glaze the coating. This method may be used to coat articles which have previously been given a metallising protective treatment by the same or another method, the final treatment being either for the purpose of enhancing the general protection or to repair parts of the previously formed protective layer which have become damaged or removed.

In another method of making articles according to the Method I An article made from niobium is subjected to a vapour degreasing process and a vapour blasting operation with a mild abrasive to obtain a fine matt surface.

After degreasing and vapour blasting, the article, which may already have had a diffusion treatment with silicon and/or other metals, is dipped in a slurry composed of the metal constituents of the glaze, including silicon, vanadium and/or manganese, in the form of powder passing a 200 mesh per inch sieve in admixture with a suspensionpromoting agent such as modified bentonite and a binder such as acrylic resin in toluene, The dipping is repeated with intermediate drying, as necessary to produce a dry coating from 0.003 inch to 0.005 inch thick. Alternatively the slurry may be sprayed onto the article by means of an air-operated spray gun, with drying between coatings as necessary.

The article with the dry adherent coating is then heated in a vacuum better than 10- millimeters of mercury, or in argon, for 3 hours at 1350 C., or at a somewhat higher temperature for a shorter period, to cause the coating to become fused to and partly difiused into the article, this action being assisted by the presence of the silicon and vanadium and/or manganese. Finally, to

Percent Silicon. Vanadium- Manganese.. Tantalum.

Tungsten Molybdenum Chromium. Titanium Zirconium Aluminium Boron The proportions of metals in the slurry correspond to those of the metal in the final glaze, subject to diffusion of metals from the underlying article. By varying the proportions of the constituents of the slurry, it is possible to control the glaze reform temperature of the glaze. The significance of this is that if the glaze becomes cracked or holed, then the newly exposed surface, which may be metallic or mixed metals and oxides, will automatically oxidize and reglaze as soon as the article during use is raised to a temperature above the glaze reform temperature. The glaze reform temperature of the metals of slurry A is about 1200 C., while the glaze reform temperature of the metals of slurry B is about 1400 C.

Articles made by this process have been found to have a life of from 150 and 200 hours at 1300 C. in oxidising gases.

Articles made from tantalum may be treated in the same way except that the content of tantalum in the slurry is replaced by niobium, so as to maintain the same degree of complexity of the glaze.

Similarly, when treating articles made from niobiumbase or tantalum base alloys containing metals such as tungsten and titanium which it is desired to have present in the glaze, it is not always necessary to include these metals in the slurry. By including a proportion of these metals, or some of them, in the slurry, it is possible however to increase the proportions in which they occur in the glaze.

Method II This method is similar to Method I except that the adherent coating is applied by a hot spraying process, for example by flame spraying or plasma arc spraying. Furthermore, the initial heating in vacuum or argon can be dispensed with; fusing, diffusing and glazing being effected by a single heating period in air for about 3 hours at 1300 C.

Spraying, hot or cold, is suitable for articles of simple shape such as gas turbine blades, while dipping is preferable for articles of complex shape such as combustion chambers of gas turbine engines.

Method III After degreasing and vapour blasting, an article made from niobium is packed in a mixture of metal powders of about 150-200 mesh per inch particle size with about 0.5% by weight of potassium fluoride contained in a refractory container made for example of alumina, sillimanite or mullite, and having a loose fitting lid. The metal powder constituents comprise, by weight, 33% silicon, 7% vanadium, 7% manganese, 7% tantalum, 22% tungsten, 7% molybdenum, 7% chromium, titanium and 5% aluminium.

The container is then heated in a vacuum better than 10* millimeters of mercury at a temperature of 1350 C. for three hours, or for a somewhat longer period at 1300" C., so that the metal powder constituents diffuse into the article. The article is thus formed with a modified surface layer comprising niobium from the article in association with the metal constituents of the treatment powder.

After the container has cooled to handling temperature, the article is removed and cleaned by brushing off the loose powder and washing in hot water. The article is then heated in air for l to 3 hours at a temperature of 1300 C. until the surface becomes oxidised and the oxides fuse to form an outer surface layer in the form of a glaze.

Method IV This method differs from Method III in that, after cleaning and vapour blasting, but before subjection to the diffusion and glazing treatments described, a surface layer of the article is modified by diffusion of titanium into it. It has been found that this pretreatment assists the subsequent formation of a uniform glaze.

The diffusion of titanium into the article is preferably carried out by heating in vacuum substantially in the manner already described except that the complex metal powder is replaced by titanium powder of about 40 mesh per inch particle size, and the heating is at about ll60. C. for 5 to 10 hours.

Whereas in Methods I and II the various constituents of a bath or spray may lie within the same ranges as those specified for the metal constituents of the glaze, the mixtures used in Methods III and IV are subject to reduced upper limits for some metals, namely Percent Manganese 15 Tantalum 25 Niobium 25 Tantalum and niobium together 25 Tungsten 10 Molybdenum 10 Chromium 10 Titanium 10 Zirconium l0 Aluminium 15 Boron 10 We claim:

1. A method of producing a refractory article composed principally of a substance selected from the group consisting of niobium, tantalum and alloys comprising at least 50% by weight of a substance selected from the the group consisting of niobium and tantalum and having a surface protected by a mixed metallic oxide glaze, the metal constituents of which are the following metals within the ranges specified (reckoned by weight on the total metal content of a sample of the glaze).

(a) silicon 30 to 50%;

(b) a metallic material consisting of at least 5% of at least one substance selected from the group consisting of vanadium and manganese, the amount of said metal constituent totalling not more than 25%;

(c) at least 3 of the following: Percent tantalum 5 to 50 niobium 5 to 50 tungsten 3 to 25 molybdenum 3 to 25 chromium 3 to 25 titanium 3 to 25 zirconium 3 to 25 aluminium 3 to 25 boron 3 to 15 subject to the limitation that the total of tantalum and niobium does not exceed 50% in which metal constituents are applied in a mixture as an adherent coating to the surface of article to be protected and the article is then heated in an oxidizing atmosphere to fuse the coating onto the article and to oxidize and glaze at least an outer surface layer of the coating, the metal constituents in the mixture being those required in the final glaze, less any which diffuse into the coating from the body of the article.

2. A method as claimed in claim 1 in which the coated article is heated in a non-oxidizing atmosphere before it is heated in an oxidizing atmosphere.

3. A method as claimed in claim 1 in which the glaze contains both niobium and tantalum.

4. A method as claimed in claim 1 in which (b) is vanadium 5 to 25%, or both vanadium and manganese totalling to 25%.

5. A method of making a refractory article composed principally of a substance selected from the group consisting of niobium, tantalum and alloys comprising at least 50% by weight of a substance selected from the group consisting of niobium and tantalum and having a surface protected by a mixed metallic oxide glaze, the metal constituents of which are the following metals within the ranges specified (reckoned by weight on the total metal content of a sample of the glaze):

(a) silicon 30 to 50%;

(b) a metallic material consisting of at least 5% of at least one substance selected from the group consisting of vanadium and manganese, the amount of said metal constituent totalling not more than 25%;

(c) at least 3 of the following: Percent tantalum 5 to 50 niobium 5 to 50 tungsten 3 to 25 molybdenum 3 to 25 chromium 3 to 25 titanium 3 to 25 zirconium 3 to 25 aluminum 3 to 25 boron 3 to subject to the limitation that the total of tantalum and niobium does not exceed 50% in which the constituent metals of the glaze, less any which diffuse from the body of the article, are diffused into the surface of the article to form a modified surface layer and the article is then heated under oxidizing conditions to convert the metals in at least an outer surface layer to oxides and to fuse the oxides to form a glaze.

6. A method according to claim 5, in which, before diffusion of the metal constituents, a surface layer of the article is modified by diffusion of titanium into it.

7. A method of producing a refractory article composed principally of a substance selected from the group consisting of niobium, tantalum and alloys comprising at least 50% .by weight of a substance selected from the group consisting of niobium and tantalum and having a surface protected by a mixed metallic oxide glaze, the metal constituents of which are the following metals within the ranges specified (reckoned by weight on the total metal content of a sample of the glaze):

(a) silicon 30 to 50%;

(b) a metallic material consisting of at least 5% of at least one substance selected from the group consisting of vanadium and manganese, the amount of said metal constituent totalling not more than 25% (c) at least 3 of the following: Percent tantalum 5 to 50 niobium 5 to 50 tungsten 3 to 25 molybdenum 3 to 25 chromium 3 to 25 titanium 3 to 25 zirconium 3 to 25 aluminium 3 to 25 boron 3 to 15 subject to the limitation that the total of tantalum and niobium does not exceed 50% in which metal constituents are applied in a mixture as an adherent coating to the surface of article to be protected and the article is then heated in a non-oxidizing atmosphere to fuse the coating onto the article and thereafter in an oxidizing atmosphere to oxidize and glaze at least an outer surface layer of the coating, the metal constituents in the mixture being those required in the final glaze, less any which diffuse into the coating from the body of the article. 8. A method as claimed in claim 7 in which the glaze contains both niobium and tantalum.

9. A method as claimed in claim 7 in which (.b) is vanadium 5 to 25 or both vanadium and manganese totalling 10 to 25 References Cited UNITED STATES PATENTS 2,809,127 10/1957 Gibson 11771 3,037,883 6/1962 Wachtell et al. 148-6.3 X 3,293,068 12/1966 Bradley et a1. 117106 X 3,317,343 5/1967 Jefferys 117107.2

FOREIGN PATENTS 676,913 8/1952 Great Britain.

ALFRED L. LEAVITT, Primary Examiner. THOMAS E. BOKAN, Assistant Examiner.

US. Cl. X.R. 29-195; 11746, 71, 106, 135.1; 1486.3 

